This invention relates to a novel liquid crystal compound which is specially useful as an electrolyte, a mixture or composition containing the compound, an electrolyte comprising a liquid crystal compound including the novel compound, and an electrochemical cell using the electrolyte, particularly a photo-electrochemical cell.
The electrolyte used in an electrochemical cell, such as a lithium ion secondary battery or a solar cell having a dye-sensitized semiconductor layer, is a medium which contains ions meeting the purpose and functions as an ion conductor transporting the ions between electrodes. A solution type electrolyte (i.e., electrolytic solution) has been of general use because of its high ion conductivity but is disadvantageous in that the solvent tends to evaporate or the solution tends to leak while used in a battery, causing reduction of battery life. Further, a metallic container should be used to seal in the electrolytic solution, which increases the weight of the battery and narrows the freedom of design.
In order to overcome the disadvantages of a solution type electrolyte, various types of electrolytes have recently been proposed. Although a gel electrolyte comprising a polymer matrix impregnated with an electrolytic solution, not being so inferior in ion conductivity, offers a level of battery performance equal to that of a solution type electrolyte, it cannot completely avoid evaporation of the solvent. A polymer electrolyte comprising a polymer, such as polyethylene oxide, having a salt dissolved therein is expected to settle the problems associated with a solution type electrolyte but has not attained sufficient ion conductivity. A molten salt, which is liquid at room temperature, such as an imidazolium or pyridinium salt having BF4xe2x88x92, (CF3SO2)2Nxe2x88x92, etc. as a counter anion, has been suggested as an electrolyte of lithium ion batteries. However, mechanical strength and ion conductivity of an electrolyte conflict with each other. When the mechanical strength of the molten salt electrolyte is enhanced by increasing the viscosity of the molten salt itself or by incorporating a polymer, a reduction in ion conductivity results. Further, these state-of-the-art electrolytes show large temperature dependence of ion conductivity, and, in particular, their ion conductivity is insufficient at low temperature.
On the other hand, study of photovoltaic power generation (i.e., solar electricity generation) consisting in conversion of light energy to electrical energy has been directed chiefly to development of monocrystalline silicon solar cells, polycrystalline silicon solar cells, amorphous silicon solar cells, and compound solar cells using cadmium telluride, copper indium selenide, etc., and some of them have been put to practical use. It is required for spread of solar cells to overcome such difficulties as a high production cost, a short supply of raw materials, and a long energy payback time. Although many solar cells using organic materials have been proposed aiming at an increase of working area and a reduction of cost, they have a low conversion efficiency and poor durability.
Under these circumstances, Nature, vol. 353, pp. 737-740 (1991) and U.S. Pat. No. 4,927,721 disclosed a photoelectric conversion device using a dye-sensitized oxide semiconductor (hereinafter, called a dye-sensitized photoelectric conversion device) and a photo-electrochemical cell comprising the device. The disclosed cell comprises a photoelectric conversion device functioning as a work electrode (negative electrode), a charge transporting layer, and a counter electrode (positive electrode). The photoelectric conversion device comprises an electrically conductive substrate and a photosensitive layer containing a semiconductor having a dye adsorbed on the surface thereof. The charge transporting layer comprises a redox system and serves for charge transport between the negative electrode and the positive electrode. The photo-electrochemical cell proposed in the above U.S. Patent is of wet type using, as a charge transporting layer, an aqueous electrolytic solution containing such a salt as potassium iodide. This system is promising in that a relatively high energy conversion efficiency (i.e., photoelectric conversion efficiency) can be reached for its low price. The outstanding problem of conventional dye-sensitized photoelectric conversion devices is that the cell will be short of the electrolytic solution due to evaporation in long-term use only to have a seriously deteriorated photoelectric conversion efficiency or fail to function as a cell.
To solve this problem, WO95/18456 teaches use of an imidazolium salt that is a low-melting compound as an electrolyte so as to prevent a shortage of an electrolytic solution. According to this method, water or an organic solvent that has been used as a solvent for an electrolyte is not necessary at all or used in a reduced amount so that improvement in durability can result. However, the durability is still insufficient and, besides, an increase in concentration of the imidazolium salt is accompanied by a reduction of photoelectric conversion efficiency. Use of a triazolium salt as an electrolyte has also been proposed, which has the same problems as the imidazolium salt.
An object of the present invention is to provide a novel liquid crystal compound and a mixture or composition containing the same which are specially useful as a material of an electrolyte for an electrochemical cell.
Another object of the present invention is to provide an electrolyte having high electrical conductivity and durability which is specially suited for use in a photo-electrochemical cell.
Still another object of the present invention is to provide an electrochemical cell having excellent durability, particularly a photo-electrochemical cell excellent in photoelectric conversion efficiency and durability.
The above objects of the present invention are accomplished by:
(1) A liquid crystal compound represented by formula (I-1) or (I-2): 
xe2x80x83wherein Q111 represents a divalent linking group with the proviso that Q111 does not represent a polymethylene group nor a phenylenebis(poly)methylene group without containing a hetero atom; X111xe2x88x92represents an anion; R141, R151, R161, and R171 each represent a hydrogen atom or a substituent; R131 represents a substituted or unsubstituted alkyl group or a substituted or unsubstituted alkenyl group; Y111 represents a divalent 4-, 5-, 6- or 7-membered ring or a divalent condensed ring composed of 4-, 5-, 6- or 7-membered rings; Q121 and Q131 each represent a divalent linking group or a single bond; n represents 1, 2 or 3; when n is 2 or 3, the plural Y111""s Q121""s, and Q131""s may be the same or different.
(2) A liquid crystal mixture comprising at least two liquid crystal compounds represented by formula (I-1) or (I-2).
(3) A liquid crystal composition comprising at least one liquid crystal compound represented by formula (I-1) or (I-2) and at least one compound selected from an alkali metal salt and an alkaline earth metal salt.
(4) An electrolyte containing at least one liquid crystal compound represented by formula (IA): 
xe2x80x83wherein R111 represents a substituted or unsubstituted alkyl or alkenyl group which is bonded to the nitrogen atom either directly or via a divalent linking group; Z111 represents an atomic group necessary to form a 5- or 6-membered aromatic heterocyclic cation together with the nitrogen atom; R121 represents a substituent serving to exhibit liquid crystal properties which contains at least one divalent cyclic linking group and at least one substituted or unsubstituted alkyl or alkenyl group; and X111xe2x88x92represents an anion, the compound represented by formula (IA) being preferably represented by formula (I-1) or (I-2).
(5) An electrochemical cell having the above-described electrolyte, particularly a photo-electrochemical cell comprising a semiconductor responsive to a radiant ray and a charge transporting layer containing the above-described electrolyte.
According to the present invention, an electrolyte which is substantially nonvolatile and excellent in charge transporting performance can be provided. The electrolyte of the present invention provides a photo-electrochemical cell which exhibits excellent photoelectric conversion characteristics and undergoes little deterioration in performance with time.